This invention relates generally to isotactic propylene homopolymer compositions and to methods for their production and use.
Multiple stage polymerization processes are known in the art as is the use of metallocene catalyst systems. Multiple stage polymerization processes, such as two-stage polymerization processes, are generally used to prepare block copolymers which contain rubbery materials. Two-stage polymerization process products may include propylene block copolymers. In some instances, the propylene/ethylene copolymer portion of these block copolymers may be rubbery. In these instances, these products may be more suitable for molding applications rather than films. In other instances, two or more metallocenes may be used for the preparation of isotactic propylene polymers.
Related patents and patent applications include: U.S. Pat. Nos. 5,280,074, 5,322,902, 5,346,925, 5,350,817, 5,483,002 and Canadian Patent Application No. 2,133,181
It has been discovered that isotactic propylene homopolymer compositions may be made by polymerizing propylene in one stage using a metallocene catalyst system and then in a separate stage using the same catalyst system to further polymerize the polymer but to a different molecular weight. The different molecular weights are produced by varying the concentration of a chain transfer agent such as hydrogen.
The resulting polymers have surprisingly high molecular weight and broad molecular weight distribution, and offer processability benefits in many applications but particularly in oriented film applications. Films made from these unique polymers have a significantly broader processability range and can be evenly stretched at lower temperatures compared to the polypropylene films available today. The resulting films have a favorable balance of properties including high strength, good optical properties, excellent shrinkage and good barrier properties.
As such, this invention relates to a propylene polymer composition which includes an isotactic propylene homopolymer with a molecular weight distribution in the range from about 2.5 to about 20.0 having hexane extractables of less than 1.0 weight percent. When the propylene polymer composition is formed into a film, the biaxially oriented film properties further characterize this propylene polymer composition. For example, the propylene polymer film, having pre-stretched dimensions of 50.8 mmxc3x9750.8 mmxc3x9720 mil, exhibits an even stretch when stretched to a final stretched thickness of about 0.75 mil between the temperature ranges of from 151.7xc2x0 C. to 157.2xc2x0 C. on a T. M. Long biaxial stretching apparatus. Before stretching, the film is preheating for 27 seconds at the stretching temperature. The film is stretched at a rate of 76.2 mm/sec.
In another embodiment, the propylene polymer composition may include a blend of first and second propylene homopolymers. The first propylene homopolymer may have a melt flow rate in the range of 0.15 dg/min to 4.0 dg/min and a molecular weight distribution in the range of 1.8 to 2.5. The second propylene homopolymer may have a melt flow rate in the range of 5 dg/min to 1000 dg/min and a molecular weight distribution in the range of 1.8 to 2.5.
In another embodiment, the propylene polymer composition includes isotactic propylene homopolymer with a molecular weight distribution in the range from about 2.5 to about 20.0, hexane extractables of less than 1.0 weight percent, a melting point greater than 145xc2x0 C., and a melt flow rate in the range of 0.2 dg/min to 30.0 dg/min. This propylene polymer composition further includes a blend of first and second propylene homopolymers. The first propylene homopolymer may have a melt flow rate in the range of 0.15 dg/min to 4.0 dg/min and a molecular weight distribution in the range of 1.8 to 2.5 and may comprise from 40 percent to 80 percent of the propylene polymer composition. The second propylene homopolymer may have a melt flow rate in the range of 5 dg/min to 1000 dg/min and a molecular weight distribution in the range of 1.8 to 2.5 and may comprises from 20 percent to 60 percent of the propylene polymer. When this propylene polymer composition is formed into a film, the biaxially oriented film properties further characterize this propylene polymer composition. For example, the propylene polymer film, having pre-stretched dimensions of 50.8 mmxc3x9750.8 mmxc3x9720 mil, exhibits an even stretch when stretched to a final stretched thickness of about 0.75 mil between the temperature ranges of from 151.7xc2x0 C. to 157.2xc2x0 C. on a T. M. Long biaxial stretching apparatus. Before stretching, the film is preheating for 27 seconds at the stretching temperature. The film is stretched at a rate of 76.2 mm/sec.
The invention further relates to processes for polymerizing isotactic polypropylene. In one embodiment, this process includes (a) polymerizing propylene in the presence of a metallocene and a first concentration of chain transfer agent sufficient to produce a first propylene homopolymer having a melt flow rate in the range from 0.15 dg/min to 4.0 dg/min and (b) polymerizing propylene in the presence of the first propylene homopolymer in the presence of a second concentration of chain transfer agent sufficient to produce the isotactic polypropylene having a molecular weight distribution in the range of from about 2.5 to about 20. The first propylene homopolymer may have a molecular weight distribution in the range of 1.8 to 2.5. Additionally, step (a) may be performed in a first reactor and step (b) may be performed in a second reactor. Furthermore, the metallocene may be a single metallocene and the single metallocene may also be present in step (b). A second propylene homopolymer having a molecular weight distribution in the range of 1.8 to 2.5 and a melt flow rate in the range from 5 dg/min to 1000 dg/min may be produced in step (b). The chain transfer agent may be hydrogen.
In another embodiment, the process for polymerizing isotactic polypropylene includes (a) polymerizing propylene in the presence of a metallocene and a first concentration of chain transfer agent sufficient to produce a first propylene homopolymer having a melt flow rate in the range from 5 dg/min to 1000 dg/min and (b) polymerizing propylene in the presence of the first propylene homopolymer in the presence of a second concentration of chain transfer agent sufficient to produce the isotactic polypropylene having a molecular weight distribution in the range of from about 2.5 to about 20. The first propylene homopolymer may have a molecular weight distribution in the range of 1.8 to 2.5. Step (a) may be performed in a first reactor and step (b) may be performed in a second reactor. The metallocene may be a single metallocene and be present in step (b). A second propylene homopolymer having a molecular weight distribution in the range of 1.8 to 2.5 and a melt flow rate in the range from 0.15 dg/min to 4.0 dg/min. may be produced in step (b). The chain transfer agent may be hydrogen.
In still another embodiment, the process includes (a) homopolymerizing propylene in the presence of a single metallocene and a first concentration of chain transfer agent sufficient to produce a first propylene homopolymer having a melt flow rate in the range from 0.15 dg/min to 4.0 dg/min and a molecular weight distribution in the range of 1.8 to 2.5 and (b) homopolymerizing propylene in the presence of the first propylene homopolymer and the single metallocene in the presence of a second concentration of chain transfer agent sufficient to produce a second propylene homopolymer having a molecular weight distribution in the range of 1.8 to 2.5 and a melt flow rate in the range from 5 dg/min to 1000 dg. The resulting isotactic polypropylene is a blend of the first and second homopolymer having a molecular weight distribution in the range of from about 2.5 to about 20 and wherein the first homopolymer comprises from 40 percent to 80 percent of the isotactic polypropylene and the second homopolymer comprises from 20 percent to 60 percent of the isotactic polypropylene. The chain transfer agent in at least one of the steps (a) and (b) is hydrogen.
In still another embodiment the process for polymerizing isotactic polypropylene includes (a) polymerizing propylene in the presence of a metallocene and a first concentration of chain transfer agent sufficient to produce a first propylene homopolymer having a first melt flow rate and a first molecular weight distribution in a first range and (b) polymerizing propylene in the presence of the first propylene homopolymer in the presence of a second concentration of chain transfer agent sufficient to produce a second propylene homopolymer having a second melt flow rate and a second molecular weight distribution in a second range wherein the second range is substantially similar to the first range such that the blend of the first and second propylene homopolymers forms the isotactic polypropylene having a molecular weight distribution in the range from about 2.5 to about 20.